Obesity is a serious illness, with a complex etiology, which is associated with increased risk in the development of multiple other morbidities including Type II diabetes, cardiovascular disease, hypertension and some neoplasias. In the United States, rates of obesity have been increasing and at present more than 50% of the population are either overweight or obese. The causes of obesity are poorly understood, however it is clear that molecular signaling pathways in the brain are important as rare cases of human obesity are caused by defective signals in these pathways. We have found that one neuropeptide, melanin-concentrating hormone (MCH), is an important regulator of energy balance and have reported that genetic ablation of MCH leads to a lean phenotype. Furthermore mice lacking MCH (MCH-/-) are resistant to diet induced obesity. Similarly, others reported that ablation of the rodent MCH receptor, MCHR-1, also leads to leanness. We have also found that MCH overexpression is associated with increased adiposity. We generated a model lacking both the MCH gene and leptin, MCH-/- ob/ob, which provided significant insights into the mechanisms by which MCH regulates body weight. In addition to effects on feeding MCH appears to have effects on energy expenditure and locomotor activity. In this proposal we will complete our characterization of the abnormalities of the MCH-/- ob/ob compared to ob/ob animals. Furthermore we will explore the hypothesis that we developed that MCH acts to suppress autonomic activity by evaluating nor-epinephrine turnover in mice lacking MCH as well as mice lacking both MCH and leptin. We will directly test the hypothesis that MCH mediates effects of leptin on sympathetic activity by examining NE turnover in normal and MCH-/- MCH-/- ob/ob and amice. We will also examine the hypothesis that we have developed that MCH acts to regulate dopaminergic signaling. To do this we will assess Dl and D2 dopamine receptors, dopamine transporters and dopamine levels in MCH-/-, MCH -/- ob/ob mice and compare these to wild type mice. We will evaluate behavioral responses to pharmacologic agents mediated through dopaminergic pathways. We will also perform microdialysis experiments to evaluate dopamine activity in freely moving mice in real time. Understanding these pathways will help understand mechanisms by which humans gain excess weight and are important in developing treatments for the prevention and treatment of obesity.